Sendai: Interactive aftershock map

Aftershocks have ravaged the east coast of Japan since the Sendai megaquake of 11 March, and are expected to continue in the coming days. Could Tokyo, with a population of 8.5 million, be hit?

The Japan Meteorological Agency announced yesterday that three aftershocks with a magnitude over 7 – and 48 shocks over magnitude 6 – have hit Japan since last Friday's magnitude 9 quake. According to the JMA, this aftershock activity is 'very high' by historical standards.

Satoko Oki at the University of Tokyo's Earthquake Research Institute says there is some evidence that last week's quake and the ensuing aftershocks triggered two earthquakes to the west of Tokyo – one in Nagano prefecture on 12 March and one in Shizuoka prefecture on 15 March.

And with so few historical examples of magnitude 9 earthquakes, we are effectively in unchartered territory. It is unknown when – or if – the likelihood of Friday's quake triggering an event in Tokyo will decrease, says Oki.

However, it does at least appear that Tokyo may dodge the bulk of the intense aftershock activity. The map above, produced by Paul Nicholls at the University of Canterbury in New Zealand, shows all of the aftershocks that have occurred in the Japanese area since last Friday. It is constantly updating by pulling data on the size and location of aftershocks from the US Geological Survey's website. Aftershocks are occurring as the tectonic plates readjust to find a new stable distribution of regional stresses after the megaquake.

The map shows that, so far, almost all of the aftershocks have occurred along the subduction zone that lies on the boundary of the Pacific plate.

Tokyo is thought to be at greatest risk from activity on the Tokai fault to the southwest of the city. This fault is, in fact, an extension of the Nankai Trough – which lies on a different subduction zone: between the Eurasian and Philippine plates. Seismologist Kevin McCue, who runs a company called the Australian Seismological Centre in Canberra, says this plate boundary 'doesn't seem to have been activated by the Sendai rupture'.

This evidence alone isn't enough to suggest that Tokyo is safe – after all, neither Nagano nor Shizuoka lie on the Pacific plate boundary. But the city does appear to be dodging the unprecedented barrage of aftershocks that continue to rattle Japan.